This invention generally relates to the frequency modulation of a carrier signal and more particularly is directed to an improved system and method for generating a stabilized frequency modulated carrier signal.
Television signal transmission involves the amplitude modulation of one carrier for video transmission and frequency modulation of a second carrier for audio transmission. The audio carrier signal is 4.5 MHz above the video carrier which, in turn, is 1.25 MHz above a lower boundary of a particular channel. Each channel is 6 MHz in width.
Subscription television (STV) systems, which involve the transmission of encoded video and audio carriers with subcarrier signals and their decoding at a receiving site, are becoming increasingly popular and more numerous. Generally, the decoded video and audio signals are remodulated on an RF channel frequency, typically in the VHF spectrum such as channels 3, 4 or 6.
In general, the audio carrier, which is transmitted by the STV system to the STV decoder at a reference frequency, is detected by means of a discriminator circuit which provides an output signal made up of a first audio signal along with an FM subcarrier. The FM subcarrier is applied to a second discriminator which provides a second audio signal. The second audio signal usually represents the desired audio, but the decoder has the facility to switch in any combination of the first or second audio signal to the input of the VCO. The output of the VCO provides the new unscrambled sound carrier to be added to the unscrambled video for modulation to the desired unused channel.
Television receiver performance is degraded if the remodulated audio carrier is not centered on the discriminator characteristic of the television receiver's audio detector. Thus, prior art attempts to improve audio performance in the television receiver have centered on improving the frequency stability of the audio carrier provided to the television receiver. An example of an STV system for recovering the audio carrier in which the various signals are separated by filtering and then recombined by signal multiplication is provided in U.S. Pat. No. 4,145,717 to Guif et al. Another approach has been to utilize a stable local reference frequency to which the generated audio carrier is compared for providing a stable signal to the television receiver. This approach unfortunately requires the use of an expensive crystal oscillator for generating the reference frequency.
Other techniques involving phase locked loops to stabilize the average frequency of the generated audio carrier often require frequency dividers in addition to the local stable crystal reference oscillator. This is necessary because phase lock can only reliably occur if the phase deviation of the signals to be phase locked are reduced to less than 360.degree.. A rather inexpensive circuit that can be used to generate an FM signal uses a VCO with LC elements for determining the center frequency. Unfortunately, these circuits have temperature drifts which result in frequency shifts that are much greater than those acceptable in TV sound modulation circuits. Attempts to stabilize such an oscillator by simple negative feedback from a frequency discriminator only result in shifting the burden of stability to the discriminator. The discriminator then is required to use expensive means for stabilization, and the overall expense and complexity of the system is increased.
In the present invention frequency drift and instability in the discriminator is eliminated by sequentially processing the reference frequency and the voltage controlled oscillator frequency to be stabilized through the same discriminator. Subsequent processing of these signals removes common mode frequency drift and instability while the difference in these average frequencies is measured and compensated for.